Wireless power transmission, which is also called non-contact charging technology, is applied to various products today. Known forms of wireless power transmission are using electromagnetic induction and radio waves. Wireless power transmission via electromagnetic induction is triggered by the discovery of Faraday's law in 1831, followed by the invention of the transformer in 1836. The transmission distance in the electromagnetic induction-type wireless power transmission is several mm or less, and the operating frequency is several hundred KHz or less in many cases. The electromagnetic induction-type is characterized in that the transmission distance is short and there is no leakage of the magnetic field, though the amount of power transmission depends on design. The wireless power transmission requires no connector and allows easy enhancement of waterproofness, and it is used in products such as chargers of electric toothbrushes, shavers and the like, for example.
On the other hand, wireless power transmission via radio waves (microwaves) has been under continuous research and development since confirmation of the existence of electromagnetic waves by Hertz in 1888 and the wireless power transmission experimental design (150 kHz, 1 MW) by Tesla in 1904. In recent years, the radio wave (microwave)-type wireless power transmission is applied also to wireless article management systems (RF-ID) and the like. Because the radio wave (microwave)-type uses radio waves, the electromagnetic field radiates outward and energy is scattered. Therefore, this type is characterized in that while long distance power transmission (up to several tens of meters) is possible, the transmission efficiency for power receiving is as low as several % or less. Note that the transmission efficiency is the ratio of the power transmitted by a transmitter and the power received by a receiver, which is the power transmission rate in a space.
The main demand for the wireless power transmission technology is to transmit power over longer distances with higher efficiency. In 2006, the wireless power feeding technology via magnetic resonance was proposed in Massachusetts Institute of Technology (MIT) (Patent Literature 1). In the magnetic resonance system, a resonance phenomenon is produced by placing two coils having high inductance (L) in each of the transmitting side and the receiving side to realize power transmission over longer distances with higher efficiency.
In the magnetic resonance wireless power transmission, the transmission efficiency of about 45% is achieved when the resonance frequency is 10 MHz and the distance between the coils is 2 m (Non Patent Literature 1). The transmission efficiency of the whole power transmission system is about 15%, which is the product in the whole system including the efficiency of a power transmitting device (mainly, an amplifier), 37.5%, and the efficiency of a power receiving device (mainly, a rectifier), 90%. Thus, when the input power to the power transmitting system is 400 W, it is possible to light a lamp of 60 W at 2 m away. In this manner the magnetic resonance method can feed power over longer distances with relatively high efficiency compared with the electromagnetic induction method.
As the electromagnetic induction system, an example where a power receiving body is placed within an electromagnetic field created by power transmitting and receiving coils is disclosed (Patent Literature 2). In this example, an AC power supply device is connected to each of the power transmitting and receiving coils, and switching of power transmission and reception can be made. Further, among the energy that creates the electromagnetic field, the energy that is not received by the power receiving body is a loss. Note that, because the electromagnetic field is created by electromagnetic induction, the distance between the power transmitting and receiving coils cannot be increased, and it is an extremely short distance that is approximately 1/10 the distance in the magnetic resonance system.
Further, as the magnetic resonance system, a configuration that includes a main power transmitting device and two or more power receiving devices and further includes an auxiliary power receiving device for power recovery is disclosed (Patent Literature 3). This configuration is designed principally for magnetic resonance power feeding to the two or more power receiving devices, and a condition for receiving power is to match resonant frequencies. This example is a one-to-many magnetic resonance power feeding system from one power transmitting device to multiple power receiving devices. In this configuration, when a plurality of power receiving devices are mounted, a certain distance needs to be maintained between the respective power receiving devices. This is because if the power receiving devices are close, coupling between resonators included in the respective power receiving devices is too high, and the resonant frequency of each power receiving device deviates from a set value, which causes significant degradation of transmission efficiency to the power receiving device. Although the minimum distance between power receiving bodies at which the deviation of the resonant frequency of each power receiving device is practically negligible depends on design of a power transmitter and receiver and a power receiving device, constraints are imposed on the distance between power receiving devices in any case.